[release-1.12] Fix mtbroker-filter to proxy response headers (#7632)

* Fix mtbroker-filter to proxy response headers

* Switch to utils.PassThroughHeaders

---------

Co-authored-by: Christoph Stäbler <[email protected]>

Author: knative-prow-robot

pkg/broker/filter/filter_handler.go

@@ -308,7 +308,8 @@ func (h *Handler) writeResponse(ctx context.Context, writer http.ResponseWriter,
 			writer.WriteHeader(http.StatusBadGateway)
 			return http.StatusBadGateway, errors.New("received a non-empty response not recognized as CloudEvent. The response MUST be either empty or a valid CloudEvent")
 		}
-		writeHeaders(dispatchInfo.ResponseHeader, writer) // Proxy original Response Headers for downstream use
+
+		writeHeaders(utils.PassThroughHeaders(dispatchInfo.ResponseHeader), writer) // Proxy original Response Headers for downstream use
 		h.logger.Debug("Response doesn't contain a CloudEvent, replying with an empty response", zap.Any("target", target))
 		writer.WriteHeader(dispatchInfo.ResponseCode)
 		return dispatchInfo.ResponseCode, nil
@@ -333,7 +334,7 @@ func (h *Handler) writeResponse(ctx context.Context, writer http.ResponseWriter,
 	defer eventResponse.Finish(nil)
 
 	// Proxy the original Response Headers for downstream use
-	writeHeaders(dispatchInfo.ResponseHeader, writer)
+	writeHeaders(utils.PassThroughHeaders(dispatchInfo.ResponseHeader), writer)
 
 	if err := cehttp.WriteResponseWriter(ctx, eventResponse, dispatchInfo.ResponseCode, writer); err != nil {
 		return http.StatusInternalServerError, fmt.Errorf("failed to write response event: %w", err)